Thermal printers are commonly used to print alphanumeric characters and bar codes on a variety of printing media such as paper, label stock, tubing, etc. Thermal printers utilize a thermal printhead having formed therein a large number of thermal printing elements generally arranged in a row. The thermal printing elements are selectively heated to apply appropriate markings to the printing media, either directly or through a meltable transfer medium.
A thermal printhead can provide high-quality printing only if there is precise control of the temperature of the printing elements on the printhead. However, it can be difficult to precisely control the temperature of the printing elements because their temperature is not solely under the control of heating signals applied to the individual printing elements. Instead, the temperature of a printing element at any point in time is determined also by the energization history of the heating element as well as the current and past energization history of adjacent printing elements. In other words, for a given power of heating signal applied to a printing element, the temperature of the printing element will be higher if the printing element was energized during previous scans of the printhead and/or the adjacent printing elements are currently being energized or were energized during previous scan periods.
There have been several approaches that have been used to control the temperature of thermal printing elements. In the past, a single temperature sensor, such as a thermistor, has been used to provide an indication of the temperature of the entire printhead. However, thermistors sensing the temperature of the entire printhead at a single location are relatively slow to respond to rapid increases in the temperature of individual printing elements. The thermal feedback provided by this technique is thus too slow to precisely control the temperature of the printing elements. Also, feedback indicative of the overall temperature of the printhead is incapable of substantially assisting in the temperature control of individual printing elements. For example, if all of the printing elements on the right-hand side of a printhead are being energized while none of the thermal elements on the left-hand side of the printhead are being energized, the temperature of the right-hand side of the printhead will be significantly greater than the temperature of the left-hand side of the printhead. However, thermal feedback from a single temperature sensor is incapable of causing the control circuit to treat one side of the printhead differently from the other side of the printhead. In short, while a single temperature sensor may be helpful in providing long-term feedback of the overall temperature of the printhead, it is incapable of providing short-term feedback and feedback responding to the temperature variation between individual printing elements.
Another approach to controlling the temperature of thermal printing elements has been to record the energization history of the printing elements and use that history to control the energization of each printing element during each scan of the printhead. For example, whether a printing element was energized during each of the previous six scans of the printhead can be recorded, and the degree of energization of the printing element during the next scan of the printhead can be adjusted accordingly. Similarly, the present or past energization state of adjacent printing elements can be recorded and used to adjust the magnitude of energy applied to the printing element during the next scan of the printhead. If a given printing element was energized during previous printhead scans, or if the adjacent printing elements are energized during the present printhead scan or they were energized during previous printhead scans, then the energy applied to the printing element is reduced. Conversely, if the printing element was not energized during prior printhead scans and the adjacent printing elements were not energized during the current or previous printhead scans, then the energy applied to the printing element is increased. The use of a printing element history to control the energy applied to printing elements during a printhead scan markedly improves the control of printing element temperature. However, this energization history technique essentially provides only very short term feedback (i.e., on the order of several printhead scan times). The energization history technique can be combined with the above-described single temperature sensor technique to provide both very short-term feedback and very long-term feedback. However, even this approach does not provide optimum temperature control of printing elements because there is a relatively long period of time during which there is no thermal feedback. In other words, the energization history technique provides thermal feedback for the last several printhead scans while the single temperature sensor technique provides thermal feedback for relatively long periods of time covering a large number of printhead scans, but there is no thermal feedback covering the period between several printhead scans and a large number of printhead scans. As a result, it is very difficult to precisely control the temperature of printing elements using the above-described techniques.